This invention relates to a metal sheet stretch-forming machine of the type having two opposed clamping jaws. While the term “stretch-forming” is used in this application, the invention is intended to have application to any type of metal-forming machine wherein jaws have a number of adjacent grippers which are collectively curvable so as to closely conform to the shape to be imparted to the metal sheet.
The clamping jaws are formed of a series of hinged grippers that move relative to each other in such a manner as to collectively form concave, convex, or lazy-S curves. These opposed jaws are used to grip opposing ends of a metal sheet while the sheet is stretched into its yield state. While in that state, the metal sheet is formed over a die. Each of the grippers is actuated against a mechanical or electrical stop by hydraulic cylinders so that the gripped sheet can be loaded flat, then caused to assume a contour roughly in the shape of the curved surface of the die. Thus, the use of curved jaws in a stretch-forming machine saves material that would otherwise be wasted by the transition from a straight jaw's opening to the surfaces of a curved die. Each of the grippers (or groups of grippers) is controlled by a hydraulic cylinder, and the collective, accumulated motion of the hydraulic cylinders of adjacent grippers defines the curve of the jaw. The stretch-forming machine may be computer or manually controlled, or a combination of computer and manual controls may be used.
Existing stretch-forming machines, including those of applicant, such as disclosed in U.S. Pat. Nos. 5,910,183 and 6,018,970, have the capability of forming longitudinal compound curved shapes such as those used in aircraft fuselage and wing sections. Because of the compound curvature, the longitudinal centerline of the stretch-formed part is elongated before and to a greater extent than the outer edges. Thus, any further stretching of the outer edge portions of the part in an effort to achieve similar elongation of the outer edge portions results in possibly overstretching the longitudinal center portion of the part, thus increasing the possibility of material failure.
Conventional methods of achieving the required elongation at the outer edges require that the part be subjected to an intermediate annealing process once proper elongation of the center portion of the part has been achieved. This method requires removing the part from the stretch-forming machine, transporting the part to an annealing furnace, annealing the part, and returning the part to and re-installing the part on the stretch-forming machine. A second stretch-forming process is then carried out. While the edge portions may then be stretched to the proper amount, the center portion is simultaneously stretched some additional amount beyond its optimum stretch, resulting in a part with different degrees of stretch in different areas.
Ideally, stretch-forming compound curved parts should be accomplished so as to impart optimum stretch to the edge portions of the part without significantly altering the optimum stretch in the center areas of the part. Accordingly, a stretch-forming machine and process is disclosed in this application that accomplishes this result.